The present invention relates to microwave filter devices having a metal cavity with dielectric inserts.
The invention is particularly advantageous when applied to filtering in the field of satellite telecommunications.
Metal cavities have long been used to filter microwaves.
Recent research has shown that loading such metal cavities with transverse inserts in the cavity is beneficial in improving the electrical performance of the resonators constituted by the metal cavities.
In this regard, reference may advantageously be had to the following publications:
[1] R. Comte, S. Verdeyme, P. Guillon, "New concept for low loss microwave devices", Electronics Letters, Vol. 30, No. 5, Mar. 3, 1994, 1995 MTT-S Digest, Orlando, Vol. 3, pp 1535-1538;
[2] R. Comte, S. Verdeyme, P. Guillon, "Rigorous design of multimodal low losses microwave cavity", ESA workshop on advanced CAD for microwave filters and passive devices, 1995, ESTEC, pp 225-231;
[3] R. Comte, S. Gendraud, S. Verdeyme, P. Guillon, C. Boschet, B. Theron, "A high Q factor microwave cavity", 1995, MTT-S Digest, Orlando, Vol. 3, pp 1535-1538.
It might nevertheless still be thought today that, because of the distribution of the field in such cavities with dielectric inserts, they could be coupled to input and output waveguides only via metal coupling irises or coaxial probes on the side walls of the cavities and not on their end walls.
The dielectric inserts distance high electromagnetic fields from the metal end walls of the cavity and thereby confine the energy in the central part of the cavity.
The energy levels are therefore very low in the vicinity of the end walls of the cavity and this is why it might be thought that correct coupling could not be achieved via those walls.
However, a problem arises with coupling via the side walls of the cavity because of coupling of spurious resonant modes in the dielectric inserts, especially for high values of input coupling.